1. Technical Field
The present invention relates to a vertical thermal treatment apparatus of a type wherein a vertical rotational shaft passes through a shaft hole and penetrates into a reaction vessel that is designed for performing a thermal treatment on a substrate to be processed, using processing gases under vacuum environment.
2. Description of Related Art
A vertical thermal treatment apparatus is known in the art as an apparatus for subjecting a batch of semiconductor wafers (hereinafter called "wafers") to a thermal treatment, and such a vertical thermal treatment apparatus is used for procedures such as the film-formation processing called reduced-pressure CVD, oxidation, and the diffusion of impurities. In each of these apparatuses, the configuration is such that a wafer boat holding wafers in multiple layers is conveyed into a vertical reaction vessel, usually from below, and a lower aperture of the reaction vessel is closed hermetically by a lid member that supports the wafer boat.
In this type of apparatus, the wafer boat is made to rotate about a vertical shaft in order to improve the uniformity of the thermal treatment within each wafer surface, and in such a case the configuration is such that the vertical rotational shaft penetrates the lid member and the wafer boat is supported on that rotational shaft.
As shown in FIG. 6, a shaft support device is used for introducing a rotational shaft through a lid member and into the reaction vessel of a reduced-pressure CVD furnace. FIG. 6 shows a state in which a lower aperture of a manifold 11, which is provided in a lower portion of a double-walled reaction tube 10, is closed by a lid member 12. A vertical rotational shaft 13 penetrates within a metal cylindrical casing 14 that is provided on a lower side of the lid member 12, with a turntable 15 attached to the upper end thereof and a pulley 16 driven by a motor (not shown in the figure) attached to the lower end thereof. A shaft bearing 17 is provided between the cylindrical casing 14 and the rotational shaft 13, with a magnetic seal means 18 provided thereabove to form a hermetic seal between the reaction vessel (which is configured of the reaction tube 10 and the manifold 11 in this case) and the exterior.
The magnetic seal means 18 consists of a support member 18a, which surrounds the rotational shaft 13, and a magnetic fluid 18b, with the magnetic fluid 18b providing a hermetic seal between the exterior and the interior space on the side of the reaction vessel of the cylindrical casing 14. The support member 18a is configured of a plurality of magnetic path members that are U-shaped in vertical section and are arrayed in the vertical direction, and the magnetic fluid 18b provides a magnetic seal between the edge portion of each of the magnetic path members and the rotational shaft 13.
If the film-formation gases should intrude into the shaft support device during the film-formation processing with the above described prior-art configuration, the film-formation gases will come into contact with cooled portions where reaction by-products will be generated, and these will adhere to the support member 18a of the magnetic seal means 18, which will slow the rotation.
The inventors have studied the use of a common furnace for CVD and for oxidation and diffusion but the prior-art configuration raises a problem when used in common in this manner in that moisture penetrates into the shaft support device during wet oxidation and that moisture causes deterioration in the magnetic fluid 18b of the magnetic seal means 18.
In that case, it could be considered to use a configuration wherein there is no through-hole in the lid member 12 and a shaft on the inner side of the lid member 12 and a shaft on the outer side thereof are linked by magnetic coupling, instead of providing the rotational shaft 13 that passes through the lid member 12, but in this case solid members of synthetic resin are used as lubrication members in a magnetic coupling structure, which means that the load resisting characteristics thereof are poor and there is a danger of vibration in the shafts at high loads. In addition, ceramic bearings are used in a magnetic coupling structure, but such bearings expand at high temperatures and cause rubbing between adjacent bearings and there is a danger that the rubbing thereof will generate particles, so a small gap is provided in order to prevent this generation of particles, but this leads to rattling at normal temperatures.
The present invention was devised in the light of the above circumstances and the object thereof is to provide a vertical thermal treatment apparatus wherein a rotational shaft penetrates into a reaction vessel in which a film is formed under a vacuum environment, and, wherein the intrusion of gases and moisture into a shaft seal portion from the interior of the reaction vessel is restrained and adverse effects on seal materials are prevented.